In Session 7 of "Atari 2600 Programing for Newbies" I learned that 50Hz PAL has up to 242 lines of actual picture.

However, the Stella Programmer's Guide says to use 228 picture lines when doing a conversion from NTSC to PAL (see screenshot below).

Which one would make most sense to use? I guess when converting an NTSC game to PAL50, it would be even easier to use 192 visible scanlines (just like NTSC) and increase the number of lines in VBLANK and OVERSCAN accordingly. Or would that cause issues? (other than not using the full screen estate).

Are there any best practices to follow when creating a game that should run on NTSC, PAL60 and PAL50?

In the world of CRT based televisions you need to be concerned with things like Safe Area. Older TVs, like in the 70s when these guidelines for the Atari were established, had a lot of variation so the safe area was smaller. Newer CRTs are much better, so you can safely increase the size of the safe area. I recall a utility was written to test how many scanlines people could see, but my search has fallen short - somebody else might recall more about that.

For NTSC 192 was recommended back then. My games vary from 182 to 202 based on the needs of my game (Draconian needed more time for game processing so only used 182 for the game screen).

In the world of CRT based televisions you need to be concerned with things like Safe Area. Older TVs, like in the 70s when these guidelines for the Atari were established, had a lot of variation so the safe area was smaller. Newer CRTs are much better, so you can safely increase the size of the safe area. I recall a utility was written to test how many scanlines people could see, but my search has fallen short - somebody else might recall more about that.

For NTSC 192 was recommended back then. My games vary from 182 to 202 based on the needs of my game (Draconian needed more time for game processing so only used 182 for the game screen).

Exactly the information I was looking for. Thanks @SpiceWare and @TheHoboInYourRoom !